Radioactive tag markers utilized in a drilling operation are by no means new. However, they have evolved over the years, from permanent markers placed in the casing or tubing, to cobalt pellets implanted into a drilling tube sub. These reference markers are necessary to provide an exact known location for perforating and other such down-hole operations.
Although each joint of drill pipe or tubing is measured prior to insertion in the bore hole, even small measurement errors and coupling differences become cumulative over the hundreds of joints of pipe used in the string, resulting in errors of several feet in some cases. Stretch and bounce in the wire-line while logging the well also plays a large role in drilling tool location error. Therefore, it has been the accepted practice to provide a highly radioactive substance as a detectable tag-marker referencing source, located in the near vicinity of the tool joint. Thus, when the initial subsurface formation log is compared to a through tubing wire-line log, for any given well, in which a tag marking reference source has been placed near the tool joint, a high gamma count spike is detectable and can be accurately located utilizing the standard API gamma ray tubing collar locator scale.
It has been assumed in the past that the radioactive tag marker reference source must have a high gamma count to overcome the natural gamma radiation being emitted by the surrounding subsurface formations. Therefore, high gamma count elements such as cobalt with gamma counts in excess of 3600 counts/sec. are currently being used. High radiation elements, when used in well logging operations, are highly controlled by the Nuclear Regulatory Commission, the Department of Transportation and various environmental agencies, both Federal and State. Personnel must be highly trained and a paper trail must be maintained at all times for the handling of such materials to minimize human exposure to radiation. A license must also be acquired and maintained for each state through which the material passes and expensive equipment must be utilized in the handling and detection process. The necessary equipment cannot be transported quickly by air, if at all, due to these time consuming restrictions.
Accordingly, a gamma ray tag marker that can be detected by the wire-line logging operation that does not exhibit the draw-backs here above mentioned is disclosed. Namely, a less hazardous substance that can be handle without risk to personnel was developed and disclosed in U.S. Pat. No. 6,285,065. However, it has since been found that smaller quantities of the natural gamma ray substances may be used for gamma counts of between 200–1000 gamma counts/sec. than first thought possible. Thereby allowing for less complicated tag marking with greater safety at less expense.